Almost any liquid that contains particles that are not fully solubilized can be characterized as a colloidal suspension. Colloidal suspensions enjoy widespread use in applications ranging from advanced materials to drug discovery. Colloid based products include paints, inks, coatings, ceramic precursors, cosmetics, and pharmaceutical compositions. In the case of ceramics, concentrated colloidal suspensions may be fabricated into dense components by sintering.
The viscosity of colloid suspensions can vary over a wide range from free-flowing liquid to flocculated gel. Dispersants that modify viscosity are often added to colloidal suspensions. A major benefit of viscosity control is the ability to lower the viscosity of a concentrated suspension. By lowering the viscosity, a suspension may be processed through pumps, pipes, and other machinery in a simpler and more cost effective manner. By tailoring interactions between colloidal particles through the addition of a dispersant, one can alter the viscosity of colloids to make them suitable for use in a broad array of applications. Through dispersant addition, colloidal suspensions may be processed at higher solids content than would otherwise be possible.
Poly(acrylic acid) (PAA) is the polyelectrolyte dispersant most widely used for the aqueous processing of ceramics. PAA contains carboxylic acid groups, one per monomer unit, along its backbone. By adding PAA, the dispersion of the colloidal particles is increased, thus reducing aggregation or flocculation. As flocculation is reduced, viscosity decreases. Another common dispersant used in ceramics processing is poly(methacrylic acid) (PMAA). PMAA contains carboxylic acid groups and methyl substituents on the backbone.
These polyelectrolyte dispersants are believed to reduce flocculation by stabilizing the colloid particles through the negative charge generated when the carboxylic acid functional groups are deprotonated. When deprotonated or ionized, the negatively charged carboxylic groups are believed to form an electrostatic repulsive barrier between the particles that form the colloid, thus reducing flocculation. It has also been postulated that the steric requirements of the dispersant provides additional stabilization.
PAA dispersant systems become less effective at reducing viscosity when higher ionic strength colloidal suspensions are involved, especially those containing multivalent ions. It is believed that the multivalent ions interfere with the electrostatic repulsive barrier of the carboxylic groups. This may result in a decreased electrostatic repulsive barrier between the colloidal particles, thereby increasing flocculation.
Hence, dispersants have been developed that can reduce flocculation of suspended particles and thus provide lowered viscosity in high ionic strength colloidal suspensions. In particular, Lewis et al. [55] teach suspensions stabilized by comb polymer dispersants and methods of using comb polymer dispersants to regulate the stability of colloidal suspensions having a high ionic strength, including suspensions containing multivalent ions.
Comb polymers, for example PAA/poly(ethylene oxide) (PEO) have nonionizable side-chains, in addition to ionizable side-chains. In relation to polymers having only ionizable side-chains such as PAA, comb polymers markedly reduce the viscosity of high ionic strength suspensions. By modifying the structure of the comb polymer, its concentration in the colloidal suspension, and the properties of the carrier liquid, the viscosity of the suspension may be altered by several orders of magnitude. The tendency of colloidal particles to flocculate is significantly reduced in relation to dispersants having only ionizable side-chains. The results are especially beneficial when the colloidal suspensions have high ionic strength arising from multivalent ions, and/or high concentrations of monovalent ions.
While not wishing to be bound by any particular theory, it is believed that the nonionizable side-chains of the comb polymers shield the ionized side-chains from ion bridging interactions (where an ion attracts dispersant coated particles), especially from multivalent ions. The nonionizable side-chains are also believed to impart steric stabilization over an interparticle separation distance that increases with the molecular weight of the nonionizable side-chains, thus making the suspension less sensitive to changes in ionic strength. In this manner, the comb polymers are believed to maintain repulsive forces between the suspended particles, even in the presence of the multivalent ions.
Suspensions stabilized by comb polymers may serve as colloid-based inks in techniques for fabricating three-dimensional structures, for example the robotic deposition technique disclosed in Cesarano et al. [54]. In such applications, the improved dispersion of nanoparticles that is yielded by comb polymers has proven to be advantageous [52, 55]. Importantly, the above inks enable the production of three-dimensional structures with feature sizes as small as 100 microns [53]. However, current direct-write techniques, which use colloid-based inks are difficult to apply to length scales finer than 100 microns, due to problems with clogging and other flow instabilities such as filter pressing.